Starship's exhaust is CO2 and water. At scale, make the fuel from ambient CO2 and you're net-zero carbon. Water vapor in the stratosphere is a potential problem

Starship's exhaust is CO2 and water. At scale, make the fuel from ambient CO2 and you're net-zero carbon. Water vapor in the stratosphere is a potential problem but only at extreme volume, since most of the exhaust is at lower altitude. Definitely we should do the moon and orbitals too, and Starship's launch costs would make that a lot easier. Mars does have things to recommend it though: lots of local resources, gravity only 1/3 of Earth's so you can get to orbit with just the Starship upper stage, only place in the solar system besides Earth where you can grow crops on natural sunlight, close to the asteroid belt.